/Engine/Source/Runtime/Engine/Public/AnimationCompression.h
Creates a header integer with four fields: NumKeys can be no more than 24 bits (positions 0..23) KeyFlags can be no more than 3 bits (positions 24..27) bReallyNeedsFrameTable is a single bit (position 27) KeyFormat can be no more than 4 bits (positions 31..28)
static int32 MakeHeader ( const int32 NumKeys, const int32 KeyFormat, const int32 KeyFlags, bool bReallyNeedsFrameTable )